In the United States and Europe, Muslim communities rely on astronomical calculations and will thus observe Ramadan from the eve of May 15, with the first day of fasting being May 16.

Scientific predictions

Astronomers calculate that Ramadan's new moon will be born on May 15 at 11:47 GMT, but its visibility on the first night may only be possible with specialised equipment.

Since 2017, Saudi Arabia and other Arab countries have used specialised infrared digital cameras that can photograph the new moon, otherwise invisible to traditional telescopes due to their limited optical range.

If such equipment is used again this year, Saudi Arabia will likely observe the first day of Ramadan on May 16.

On the eve of May 16 however, the new Moon should be visible with the naked eye around the world, making it most likely that Pakistan and other countries, depending on their local sightings, will observe the first day of fasting on May 17.

Actual visibility of the crescent will depend on factors such as atmospheric conditions, cloudiness and the distance between the sun and the moon on the horizon.

Visibility of the crescent moon on May 15 using the Yallop criterion [UK Hydrographic Office]

Visibility of the crescent moon on May 16 using the Yallop criterion [UK Hydrographic Office]

Lunar calendar

Muslim lunar months last between 29 and 30 days, depending on the sighting of the new moon on the 29th night of each month. If the new moon is not visible, the month lasts 30 days.

To declare the beginning of Ramadan, Saudi Arabia and other Muslim-majority countries depend on the testimonies of local moon sighters. The Judicial High Court then decides when Ramadan begins.

Saudi Arabia's official Umm al-Qura calendar marks the first day of Ramadan as May 16, 2018.

By the Gregorian solar calendar, Ramadan comes 10 to 12 days earlier each year. Last year, the first day of Ramadan was on May 27, 2017.

Holy month

For Muslims, Ramadan is the month in which the first verses of the Quran, Islam's holy book, were revealed to the Prophet Muhammad more than 1,400 years ago.

During Ramadan, Muslims abstain from eating, drinking, smoking and sex from dawn to sunset. This fast is intended to bring the faithful closer to God and to remind them of the suffering of those less fortunate.

Fasting during Ramadan is one of the five pillars of Islam, along with the Muslim declaration of faith, daily prayer, charity and performing the Hajj pilgrimage to Mecca.